Method for chemical polishing of copper



United States Patent Ofiice 3,008,812 Patented Nov. 14, 1961 3,008,812METHOD FOR CHEMICAL POLISHING OF COPPER Heinz Spiihn, Darmstadt,Germany, and Frederick H. Wells, Erdington, England, assignors toAlh'right and Wilson (Mfg) Limited, Birmingham, England No Drawing.Filed July 21, 1958, Ser. No. 749,597

Claims priority, application Great Britain, Aug. 20, 1957 4 Claims. (Cl.156---20) This invention relates to the chemical polishing of copper andits alloys.

A bath for chemically polishing metal surfaces has been proposed inUnited States Patent No. 2,446,060 consisting of concentrated solutionsof from 5% to 85% by volume of nitric acid of about 1.42 specificgravity, admixed with from to 95% by volume of phosphoric acid of about1.7 specific gravity and from 0% to 90% by volume of acetic acid ofabout 1.05 specific gravity. It was stated that the maximum amount ofwater which may be added to baths containing nitric acid and phosphoricacids alone, the composition of which could be varied from to 75% nitricacid and 95% to 25% phosphoric acid, was by volume since at proportionsof water above this, the polish obtained would be impaired.

It was further stated in British Patent No. 73 8,744 that where noacetic acid was present in the bath, but only phosphoric acid and nitricacid, the presence of water is essential in order to obtain asatisfactory polish and a bath for the chemical polishing. of copper,nickel and alloys of those metals, with or without the addition of zinc,was proposed which comprises from 5% to 83% of phosphoric acid, from 2%to 50% of nitric acid, and from to 45% of water.

It has now been found that very much improved polishing is obtainedusing baths containing arsenic acid together with phosphoric acid andnitric acid. The baths of the present invention give a specular polishconsiderably superior to that obtained using any bath previously known.

According to the present invention there is provided a solution for thechemical polishing of copper and its alloyswhich comprises from 5% to40% arsenic acid (of a concentration of 80% by weight in water), from 2%to 50% nitric acid of specific gravity 1.5 and from 5% to 75% phosphoricacid of specific gravity 1.75; preferably the solution also containsglacial acetic acid and/or up to water. The foregoing percentages areall quoted by volume.

The preferred solutions contain from 10% to 30% arsenic acid,.from 20%to 30% phosphoric acid, from 30% to 50% acetic acid, from 10% to 15%nitric acid, and from 0% to 10% water, all percentages being by volumeand the strengths of the acids being as hereinbefore defined.

It has been found that the polishing rate of the solutions is controlledby the quantity of glacial acetic acid present. If no acetic acid ipresent, the polishing rate tends to be too high, and if more than 70%is present, the rate is too slow for most practical purposes. If morethan 75% of phosphoric acid is present, the metal is etched. If morethan 30% of nitric acid is present, the polish obtained, althoughbright, is not very specular, and if more than 40% of arsenic acid ispresent, the surface of the immersed metal becomes pitted.

The solutions will continue to polish without requiring regenerationuntil they contain up to about 90 grams per litre of dissolved metal.When regeneration is necessary, that is when polish is impaired or thepolishing rate is appreciably altered, it may be achieved merely by theaddition of any constituent which is found to be deficient afteranalysis of the polishing solution. It is usually sufficient to addnitric acid and acetic acid and it has not been found necessary to addfurther quantities of arsenic acid.

In use, the solutions are preferably maintained at a temperature of from50 C. to C. and with the preferred solutions of this inventionsatisfactory polish is obtained in about three minutes.

The solutions of the present invention may satiss factorily be stored invesselsmade, of such materials as glass, chemical stoneware, stainlesssteel or polyvinyl resins.

It has not been found that the presence of arsenic acid in the polishingsolution introduces any danger in the form of toxic fumes or deposits onthe polished metal surfaces. No arsine has been detected in the fumesfrom the solutions even in the absence of nitric acid and no appreciabledeposit of arsenic compounds has been found on the surface of thepolished metal.

It is to be noted that the method of chemically polishing metal surfacesaccording to the invention consists in merely immersing them, asdistinct from processes in Which the metal is made the anode in thepolishing solution as electrolyte. The baths of the present inventionare not suitable for use in electro-polishing.

The following examples serve to illustrate the manner in which theinvention may be performed, the percentages in each case being by volumeand the concentrations of the acids used being as hereinbefore stated.It is to be understood that the percentages of Water refer to addedWater, excluding the water already present in the acids before mixing.

Example 1 A solution was prepared containing 20% phosphoric acid, 15%arsenic acid, 45% acetic acid, 10% nitric acid and 10% Water, and themetal articles to be polished were immersed in it for three minutes at atemperature of 6080 C. A reasonably good polish was obtained, beingbetter on copper and nickel silver than on brass.

Example 2 A solution was prepared containing 20% phosphoric acid, 30%arsenic acid, 40% acetic acid, 10% nitric acid and no added water. Avery good polish was obtained on brass articles immersed for threeminutes at 60 80 C. A good polish was obtained on gilding metal immersedfor three minutes at 50-60 C. but no polish at all was obtained oncopper or nickel silver.

Example 3 Example 4 A solution was prepared containing 20% phosphoricacid, 30% arsenic acid, 35% acetic acid, 15 nitric acid and no addedwater. A good polish was obtained on brass articles immersed for threeminutes at 6080 C. but the metal solution rate was rather high. A fairpolish was obtained on gilding metal immersed for three minutes at 70 C.but no polish was obtained on nickel silver.

In the foregoing examples the brass used was composed of 70% by weightof copper and 30% by weight of zinc and the gilding metal was composedof from 80 %90% by weightrof copper and from 20%-10% by weight of zinc.

We claim:

1. The method of chemically polishing articles of a metal of the classconsisting of copper and alloys of copper, which comprises immersing thearticle in a solution comprising from to 30% by volume of arsenic acidof a concentration of 80% by weight in Water, from to by volume ofphosphoric acid of specific gravity 1.75, from 30% to 50% by volume ofglacial acetic acid, from 10% to 15% by volume of nitric acid ofspecificgravity 1.5 and from 0% to 10% by volume of additional water.

2. The method as claimed in claim 1 in which said solution is maintainedat a temperature in the range of from 50 C. to 80 C.

3. The method of chemically polishing articles of a metal of the classconsisting of copper and alloys of copper, which comprisesimrnersing thearticle in a solution comprising from 5% to 40% of arsenic acid of aconcentration of 80% by Weight in water, from 2% to 4 by volume ofnitric acid of specific gravity 1.5, and from 5% to by volume ofphosphoric acid of specific gravity 1.75.

4. The method as claimed in claim 3 in which the nitric acid is presentin an amount not exceeding 30% by volume.

References Cited in the file of this patent UNITED STATES PATENTS2,005,780 Gravell June 25, 1935 2,172,171 Meyer Sept. 5, 1939 2,282,351Faust May 12, 1942 2,446,060 Pray et -al July 27, 1948 2,456,947Jermstedt Dec. 21, 1948 2,650,157 Cochran Aug. 25, 1953 2,729,551 Cohn MJan. 3, 1956

1. THE METHOD OF CHEMICALLY POLISHING ARTICLES OF A METAL OF THE CLASSCONSISTING OF COPPER AND ALLOYS OF COPPER, WHICH COMPRISES IMMERSING THEARTICLE IN A SOLUTION COMPRISING FROM 10% TO 30% BY VOLUME OF ARSENICACID OF A CONCENTRATION OF 80% BY WEIGHT IN WATER, FROM 20% TO 30% BYVOLUME OF PHOSPHORIC ACID OF SPECIFIC GRAVITY 1.75, FROM 30% TO 50% BYVOLUME OF GLACIAL ACETIC ACID, FROM 10% TO 15% BY VOLUME OF NITRIC ACIDOF SPECIFIC GRAVITY 1.5 AND FROM 0% TO 10% BY VOLUME OF ADDITIONALWATER.